U.S. Pat. No. 6,755,814, which is commonly owned by the assignee of the present invention, discloses an implantable infusion pump that has a reservoir level detector. The pump includes a housing having a can and a base plate, which base plate bottom divides the housing into a pump electronic chamber and a propellant chamber. A bellow mechanism is disposed within the propellant chamber. The bellow mechanism has a base or bottom plate. The medicament for delivery to a patient is stored within the bellow mechanism. A propellant is disposed about the bellow mechanism within the propellant chamber. The propellant by compressing the bellow mechanism pushes the medicament out of the bellow mechanism through a flow restrictor, a valve and outlet. The bottom of the bellow gets closer to the base plate as the medicament flows out of the bellow. A coil is disposed within a recess in the lower (propellant facing) surface of the base plate. A capacitor is electrically connected to the coil to form a resonant circuit.
When energized, the coil generates a primary electromagnetic field, which flows through the bottom plate of the bellow mechanism and induces eddy currents therein which intensities increase the closer the bottom plate gets to the coil. The eddy currents generate a secondary magnetic field, which is coupled back to the primary field. The closer the bottom plate is to the coil, the stronger the secondary magnetic field is and its influence on the primary field. This coupling brings about change to the inductance of the coil and thus brings about a displacement or shift of the resonance frequency of the resonant circuit depending on the distance between the coil and the bottom plate. Upon measuring the resonance frequency, which is dependent upon the inductance, one skilled in the art can determine the distance that the bottom plate of the bellow mechanism is from the base plate, which can be used to determine the volume of medicament remaining within the bellow mechanism.
In the prior art, such as the apparatus disclosed in U.S. Pat. No. 6,755,814, the measurement of fluid remaining in the reservoir is only accurate for the last 20 ml of fluid within the bellow mechanism. At fluid levels greater than about 20 ml and because of the increasing distance between the coil and the bottom plate at those greater volumes, the measured inductance doesn't vary enough to provide accurate measurements. Accordingly, there is a need for an implantable infusion pump where the volume of fluid within the bellow mechanism can be measured with a greater degree of accuracy, not only above 20 ml, but also for the range from 0 to 20 ml. There is also a need for an implantable infusion pump where the volume of fluid within the bellow mechanism can be measured for the entire volume range of the reservoir, including at larger volumes, such as, for example, above 20 ml. There is also a need for an implantable infusion pump that can detect a leak or a valve blockage within the pump system.